The 26-year-old Tullis-Joyce previously played in Europe with French club Reims. United did not specify the length of the contract or value of her transfer.
"Today has been pure excitement for me. Manchester United is such a historic club, that has already done so much in the women's game. I'm so honoured to get this opportunity," she said.
United turned down a bid -- reported to be a record offer for a women's goalkeeper -- already this summer and club sources told ESPN ahead of Thursday's transfer deadline that Earps, who has been heavily linked with Arsenal, will stay this season.
Tullis-Joyce is initially likely to provide back up to Earps.
"Phallon has a wonderful array of talents, and we look forward to seeing her growth at Manchester United," United coach Marc Skinner said.